Variable gain amplifier

ABSTRACT

A variable gain amplifier includes a plurality of element circuits each having an output current that increases at a constant rate with a change in a variable control voltage, voltages that increase in steps of the change being respectively supplied to the plurality of element circuits as their reference voltages and the control voltage being supplied to each of the plurality of element circuits, multipliers for multiplying the output currents from the plurality of element circuits by one another, and an amplifier for carrying out a variable gain amplification based on an output current from the multipliers. The variable gain amplifier can suppress any change in the characteristics thereof due to temperature compensation of the characteristics and variations in manufacturing of transistors, and carry out a linear gain control operation on the control voltage when the gain is expressed in a logarithm.

FIELD OF THE INVENTION

The present invention relates to a variable gain amplifier that linearlycontrols its gain (dB) expressed in a logarithm with respect to acontrol voltage by exponentially controlling the gain with respect tothe control voltage.

BACKGROUND OF THE INVENTION

FIG. 1 is a circuit diagram showing a prior art variable gain amplifier.In the figure, reference numeral 1 denotes a variable voltage powersupply, reference numeral 2 denotes a common emitter transistor, andreference numeral 3 denotes an amplifier.

Next, the operation of the prior art variable gain amplifier will beexplained.

As shown in FIG. 1, when a control voltage V_(BE) generated by thevariable voltage power supply I linearly varies, a collector current Icpassing through the common emitter transistor 2 varies according to anexponential function of the control voltage V_(BE). By supplying thecollector current Ic that varies according to this exponential functionto the amplifier 3 as a current source, the gain of the amplifier 3 isexponentially controlled with respect to the control voltage V_(BE).

Thus, the gain (dB) of the amplifier 3 expressed in a logarithm islinearly controlled with respect to the control voltage V_(BE) byexponentially controlling the gain with respect to the control voltageV_(BE).

A relationship between the collector current Ic and the control voltageV_(BE) is expressed by the following equation (1):Ic=Is*exp((q/k*T)*V _(BE))  (1)where Is is a saturated current, q is a charge, k is the Boltzmann'sconstant, and T is an absolute temperature.

In the prior art variable gain amplifier constructed as mentioned above,the collector current Ic that varies according to the exponentialfunction of the control voltage V_(BE) is dependent upon the absolutetemperature T, as can be seen from equation (1), and thischaracteristics cannot be temperature-compensated with a high degree ofprecision.

Another problem with the prior art variable gain amplifier is that theslope of the collector current Ic vs. control voltage V_(BE)characteristics curve varies when the saturation-current Is varies dueto variations in manufacturing of the transistor 2 in above-mentionedequation (1), and this change in the characteristics due to variationsin manufacturing of the transistor 2 cannot be suppressed.

The present invention is made in order to solve the above-mentionedproblems, and it is therefore an object of the present invention toprovide a variable gain amplifier that linearly controls its gain (dB)expressed in a logarithm with respect to a control voltage bysuppressing changes in its characteristics due to temperaturecompensating of the characteristics and variations in manufacturing oftransistors included in the variable gain amplifier, and byexponentially controlling its gain with respect to the control voltage.

DISCLOSURE OF THE INVENTION

A variable gain amplifier in accordance with an aspect of the presentinvention described in claim 1 includes a plurality of element circuitseach of which has two inputs which are a reference voltage and a controlvoltage which can vary with respect to the reference voltage, and eachof which has an output current that increases at a constant rate with apredetermined change in the control voltage, voltages which increase insteps of the predetermined change in the control voltage beingrespectively supplied to the plurality of element circuits as theirreference voltages and the variable control voltage being supplied toeach of the plurality of element circuits, multipliers for multiplyingthe output currents from the plurality of element circuits by oneanother, and an amplifier for carrying out a variable gain amplificationbased on an output current obtained by the multipliers.

The output current output from the multipliers thus exhibits controlvoltage vs. output current characteristics in which the output currentvaries with respect to the control voltage according to an exponentialfunction. When the gain is expressed in a logarithm, the variable gainamplifier can carry out linear gain control with respect to the controlvoltage. Although the control voltage vs. output current characteristicsof each of the plurality of element circuits are varied dependently upontemperatures, temperature-dependent changes in the slopes of the controlvoltage vs. output current characteristics curves of any two adjacentelement circuits at a connecting point between them can be compensatedand therefore the temperature dependence of the control voltage vs.output current characteristics of the variable gain amplifier can becompensated. In addition, the control voltage vs. output currentcharacteristics can be hardly changed regardless of variations inmanufacturing of transistors in the whole of the variable gainamplifier, and any change in the characteristics of the variable gainamplifier due to variations in manufacturing of the transistors can besuppressed.

In the variable gain amplifier in accordance with another aspect of thepresent invention described in claim 2, each of the plurality of elementcircuits includes a first transistor to which the control voltage issupplied, a second transistor to which the reference voltage is suppliedand which constitutes a differential pair together with the firsttransistor, and a third transistor to which the reference voltage issupplied and which constitutes a current mirror circuit together withthe second transistor, a ratio between the second transistor and thethird transistor in size being 1:N−1 when the constant rate at which theoutput current increases is N−1, the output current being output incommon from ends of the first and second transistors and a constantcurrent source that outputs a maximum output current being connected incommon to other ends of the first through third transistors.

Therefore, each of the plurality of element circuits can have a simplestructure.

In the variable gain amplifier in accordance with a further aspect ofthe present invention described in claim 3, each of the plurality ofelement circuits includes a first transistor having an end connected toa constant current source, a second transistor which constitutes acurrent mirror circuit together with the first transistor, a thirdtransistor which constitutes a current mirror circuit together with thefirst transistor, the third transistor having an end connected to anoutput current terminal, a fourth transistor to which the referencevoltage is supplied, a fifth transistor to which the control voltage issupplied and which constitutes a differential pair together with thefourth transistor, ends of the fifth and fourth transistors beingconnected in common to an end of the second transistor, and a transistorcircuit network that diverts a part of a current from the output currentterminal in proportion to a current flowing through the fifth transistorand that makes the part of the current pass therethrough without makingit flow through the third transistor, sizes of the second and thirdtransistors and transistors included in the transistor circuit networkbeing determined so that a ratio between the diverted part of thecurrent and the current flowing through the third transistor is N−1:1when the diverted part of the current has a maximum value.

Therefore, each of the plurality of element circuits can have a simplestructure.

A variable gain amplifier in accordance with another aspect of thepresent invention described in claim 4 includes a plurality ofcascade-connected element circuits each of which has two inputs whichare a reference voltage and a control voltage which can vary withrespect to the reference voltage, and each of which has an outputcurrent that increases at a constant rate with a predetermined change inthe control voltage, an input current being supplied to a first-stageone of the plurality of element circuits, voltages which increase insteps of the predetermined change in the control voltage beingrespectively supplied to the plurality of element circuits as theirreference voltages, and the variable control voltage being supplied toeach of the plurality of element circuits, and an amplifier for carryingout a variable gain amplification based on an output current from theplurality of element circuits.

The output current output from the plurality of element circuits thusexhibits control voltage vs. output current characteristics in which theoutput current varies with respect to the control voltage according toan exponential function. When the gain is expressed in a logarithm, thevariable gain amplifier can carry out linear gain control with respectto the control voltage. Although the control voltage vs. output currentcharacteristics of each of the plurality of element circuits are varieddependently upon temperatures, temperature-dependent changes in theslopes of the control voltage vs. output current characteristics curvesof any two adjacent element circuits at a connecting point between themcan be compensated and therefore the temperature dependence of thecontrol voltage vs. output current characteristics of the variable gainamplifier can be compensated. In addition, the control voltage vs.output current characteristics can be hardly changed regardless ofvariations in manufacturing of transistors in the whole of the variablegain amplifier, and any change in the characteristics of the variablegain amplifier due to variations in manufacturing of the transistors canbe suppressed.

In the variable gain amplifier in accordance with another aspect of thepresent invention described in claim 5, each of the plurality of elementcircuits includes a first transistor to which the control voltage issupplied, a second transistor to which the reference voltage is suppliedand which constitutes a differential pair together with the firsttransistor, a third transistor to which the reference voltage issupplied and which constitutes a current mirror circuit together withthe second transistor, a ratio between the second transistor and thethird transistor in size being 1:N−1 when the constant rate at which theoutput current increases is N−1, a fourth transistor having an end intowhich an input current flows, a fifth transistor that has an endconnected in common to ends of the first through third transistors, andthat constitutes a current mirror circuit together with the fourthtransistor, and an output current circuit connected in common to otherends of the first and second transistors.

Therefore, each of the plurality of element circuits can have a simplestructure.

In the variable gain amplifier in accordance with a further aspect ofthe present invention described in claim 6, each of the plurality ofelement circuits includes a first transistor having an end into which aninput current flows, a second transistor which constitutes a currentmirror circuit together with the first transistor, a third transistorwhich constitutes a current mirror circuit together with the firsttransistor, the third transistor having an end-connected to an outputcurrent circuit, a fourth transistor to which the reference voltage issupplied, a fifth transistor to which the control voltage is suppliedand which constitutes a differential pair together with the fourthtransistor, ends of the fifth and fourth transistors being connected incommon to an end of the second transistor, and a transistor circuitnetwork that diverts a part of a current from the output current circuitin proportion to a current flowing through the fifth transistor and thatmakes the part of the current pass therethrough without making it flowthrough the third transistor, and characterized in that sizes of thesecond and third transistors and transistors included in the transistorcircuit network are determined so that a ratio between the diverted partof the current and the current flowing through the third transistor isN−1:1 when the diverted part of the current has a maximum value.

Therefore, each of the plurality of element circuits can have a simplestructure.

A variable gain amplifier in accordance with a still further aspect ofthe present invention described in claim 7 includes a plurality ofcascade-connected element circuits each of which has two inputs whichare a reference voltage and a control voltage which can vary withrespect to the reference voltage, and each of which has a gain thatincreases at a constant rate with a predetermined change in the controlvoltage, an input voltage being supplied to a first-stage one of theplurality of element circuits, voltages which increase in steps of thepredetermined change in the control voltage being respectively suppliedto the plurality of element circuits as their reference voltages, thevariable control voltage being supplied to each of the plurality ofelement circuits, and an output voltage being generated by a last-stageone of the plurality of element circuits.

Therefore, when the gain is expressed in a logarithm, the variable gainamplifier can carry out linear gain control with respect to the controlvoltage using the plurality of element circuits. Although the controlvoltage vs. gain characteristics of each of the plurality of elementcircuits are varied dependently upon temperatures, temperature-dependentchanges in the slopes of the control voltage vs. gain characteristicscurves of any two adjacent element circuits at a connecting pointbetween them can be compensated and therefore the temperature dependenceof the control voltage vs. gain characteristics of the variable gainamplifier can be compensated. In addition, the control voltage vs. gaincharacteristics can be hardly changed regardless of variations inmanufacturing of transistors in the whole of the variable gainamplifier, and any change in the characteristics of the variable gainamplifier due to variations in manufacturing of the transistors can besuppressed.

In the variable gain amplifier in accordance with another aspect of thepresent invention described in claim 8, each of the plurality of elementcircuits includes a first transistor to which the control voltage issupplied, a second transistor to which the reference voltage is suppliedand which constitutes a differential pair together with the firsttransistor, a third transistor to which the reference voltage issupplied and which constitutes a current mirror circuit together withthe second transistor, a ratio between the second transistor and thethird transistor in size being 1:N−1 when the constant rate at which thegain increases is N−1, a fourth transistor to which an input voltage issupplied, the fourth transistor having an end connected in common toends of the first through third transistors, and a resistor connectedbetween other ends of the first and second transistors and a powersupply, each of the plurality of element circuits generating an outputvoltage from between the resistor and the other ends of the first andsecond transistors.

Therefore, each of the plurality of element circuits can have a simplestructure.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a circuit diagram showing a prior art variable gain amplifier;

FIG. 2 is a block diagram showing an element circuit in accordance withembodiment 1 of the present invention;

FIG. 3 is a characteristics figure showing control voltage vs. outputcurrent characteristics of the element circuit;

FIG. 4 is a block diagram showing a variable gain amplifier;

FIG. 5 is a characteristics figure showing control voltage vs. outputcurrent characteristics of the variable gain amplifier;

FIG. 6 is a characteristics figure showing temperature dependence of thecontrol voltage vs. output current characteristics of the elementcircuit;

FIG. 7 is a characteristics figure showing the control voltage vs.output current characteristics of the variable gain amplifier at a hightemperature;

FIG. 8 is a characteristics figure showing the control voltage vs.output current characteristics of the variable gain amplifier at a lowtemperature;

FIG. 9 is a circuit diagram showing the details of an example of anelement circuit in accordance with embodiment 2 of the presentinvention;

FIG. 10 is a circuit diagram showing the details of another example ofthe element circuit;

FIG. 11 is a circuit diagram showing the details of an example of anelement circuit in accordance with embodiment 3 of the presentinvention;

FIG. 12 is a circuit diagram showing the details of another example ofthe element circuit;

FIG. 13 is a block diagram showing an element circuit in accordance withembodiment 4 of the present invention;

FIG. 14 is a characteristics figure showing control voltage vs. outputcurrent characteristics of the element circuit;

FIG. 15 is a block diagram showing a variable gain amplifier;

FIG. 16 is a characteristics figure showing control voltage vs. outputcurrent characteristics of the variable gain amplifier;

FIG. 17 is a circuit diagram showing the details of an example of anelement circuit in accordance with embodiment 5 of the presentinvention;

FIG. 18 is a circuit diagram showing the details of another example ofthe element circuit;

FIG. 19 is a circuit diagram showing the details of an example of anelement circuit in accordance with embodiment 6 of the presentinvention;

FIG. 20 is a circuit diagram showing the details of another example ofthe element circuit;

FIG. 21 is a block diagram showing an element circuit in accordance withembodiment 7 of the present invention;

FIG. 22 is a characteristics figure showing control voltage vs. gaincharacteristics of the element circuit;

FIG. 23 is a block diagram showing a variable gain amplifier;

FIG. 24 is a characteristics figure showing control voltage vs. gaincharacteristics of the variable gain amplifier; and

FIG. 25 is a circuit diagram showing the details of an example of anelement circuit in accordance with embodiment 6 of the presentinvention.

PREFERRED EMBODIMENTS OF THE INVENTION

Hereafter, in order to explain this invention in greater detail, thepreferred embodiments of the present invention will be described withreference to the accompanying drawings. Embodiment 1.

FIG. 2 is a block diagram showing an element circuit in accordance withembodiment 1 of the present invention. In the figure, reference numeral11 denotes the element circuit. FIG. 3 is a figure showing controlvoltage vs. output current characteristics of the element circuit.

FIG. 4 is a block diagram showing a variable gain amplifier. In thefigure, reference numeral 3 denotes an amplifier, reference symbols 11 ₁to 11 _(M) denote M element circuits (M is an arbitrary natural number),respectively, and reference symbols 12 ₁ to 12 _(M−1) denote M−1multipliers, respectively. FIG. 5 is a characteristics figure showingcontrol voltage vs. output current characteristics of the variable gainamplifier.

Next, the operation of the variable gain amplifier in accordance withembodiment 1 of the present invention will be explained.

As shown in FIG. 2, the element circuit 11 has signal inputs which are areference voltage Vref and a control voltage Vcont, and a signal outputwhich is a current Iout. The element circuit 11 has certain controlvoltage vs. output current characteristics in which the amount of theoutput current Iout varies from I₀ to N*I₀ (N is an arbitrary numberlarger than 1) with a predetermined change Vr in the control voltage,that is, the rate of increase in the output current is N−1 when thecontrol voltage Vcont can be varied with respect to the referencevoltage Vref, as shown in FIG. 3.

As shown in FIG. 4, M above-mentioned element circuits 11, i.e., the Melement circuits 11 ₁ to 11 _(M) are disposed in the variable gainamplifier, and voltages that increase in steps of the predeterminedvoltage change Vr in the control voltage are respectively supplied tothe plurality of element circuits 11 ₁ to 11 _(M) as their referencevoltages Vref1 to VrefM. That is, (VrefM)−(VrefM−1)=Vr. The variablecontrol voltage Vcont is supplied in common to the plurality of elementcircuits 11 ₁ to 11 _(M).

The plurality of multipliers 12 ₁ to 12 _(M−1) multiply the outputcurrents Iout of the plurality of element circuits 11 ₁ to 11 _(M) byone another to generate an output current Iout, and the variable gaincontrol of the amplifier 3 is carried out based on the output currentIout generated by the plurality of multipliers.

As a result, as shown in FIG. 5, the variable gain amplifier has controlvoltage vs. output current characteristics in which the amount of theoutput current Iout varies from I₀ ^(M), via N*I₀ ^(M), N²*I₀ ^(M), . .. , to N^(M)*I₀ ^(M) and is approximately expressed by an exponentialfunction with respect to the voltage change Vr in the control voltageVcont. When the gain of the amplifier 3 is expressed in a logarithm, thegain of the amplifier 3 can be linearly controlled with respect to thecontrol voltage Vcont.

Thus, since the variable gain amplifier does not use exponentialcharacteristics of transistors included in the variable gain amplifier,any change in the characteristics of the variable gain amplifier due tovariations in manufacturing of the transistors can be suppressed.

By setting the number of element circuits, as stages, included in thevariable gain amplifier to an appropriate number and generating theplurality of reference voltages Vrefl to VrefM with a high degree ofprecision, the slope of the control voltage vs. output currentcharacteristics curve can be hardly changed regardless of variations inmanufacturing of the transistors in the whole of the variable gainamplifier, and any change in the characteristics of the variable gainamplifier can be suppressed.

FIG. 6 is a characteristics figure showing temperature dependence of thecontrol voltage vs. output current characteristics of each of theplurality of element circuits. The slope of the control voltage vs.output current characteristics curve of each of the plurality of elementcircuits decreases with increase in the ambient temperature with respectto ordinary temperatures, and increases with decrease in the ambienttemperature with respect to ordinary temperatures.

FIG. 7 is a characteristics figure showing the control voltage vs.output current characteristics of the variable gain amplifier at a hightemperature, and FIG. 8 is a characteristics figure showing the controlvoltage vs. output current characteristics of the variable gainamplifier at a low temperature. When the plurality of element circuitsare connected as mentioned above, since at a connecting point betweenany two adjacent element circuits their control voltage vs. outputcurrent characteristics curves are shifted in upward and downwarddirections due to the temperature dependence of the control voltage vs.output current characteristics of each of the two adjacent elementcircuits, respectively, the upward and downward shifted parts of thecontrol voltage vs. output current characteristics curves can becompensated. Therefore, the temperature dependence of the controlvoltage vs. output current characteristics of the variable gainamplifier can be compensated.

Embodiment 2

FIG. 9 is a circuit diagram showing the details of an example of anelement circuit in accordance with embodiment 2 of the presentinvention, and shows the details of the element circuit 11 of FIG. 2. Inthe figure, reference symbol Q1 denotes a bipolar transistor (simplyreferred to as a transistor and also referred to as a first transistorfrom here on) having a base to which a control voltage Vcont issupplied, reference symbol Q2 denotes a transistor (i.e., a secondtransistor) having a base to which a reference voltage Vref is supplied,the transistor Q2 constituting a differential pair with the transistorQ1, and Q3 denotes a transistor (i.e., a third transistor) having a baseto which the reference voltage Vref is supplied, the transistor Q3constituting a current mirror circuit with the transistor Q2 and theratio between the emitter areas of the transistors Q2 and Q3 being 1:N−1when a constant rate at which an output current increases is N−1. Theoutput current Iout is made to flow from a connecting point of thecollectors of the transistors Q1 and Q2, and a power supply source Vccis connected to the collector of the transistor Q3. Reference symbol NI₀denotes a constant current source that makes a maximum output currentflow through the element circuit and that is connected in common to theemitters of the transistors Q1 to Q3.

Next, the operation of the element circuit in accordance with embodiment2 of the present invention will be explained.

In FIG. 9, when the control voltage Vcont is sufficiently small ascompared with the reference voltage Vref, no current flows into thetransistor Q1. In addition, since the combination of the transistors Q2and Q3 is a current mirror circuit in which the ratio between theiremitter areas is 1:N−1, a current having an amount of I₀ flows into thetransistor Q2 and a current having an amount of (N−1)*I₀ flows into thetransistor Q3. As a result, the current having the amount of I₀ flows asthe output current Iout.

When the control voltage Vcont is sufficiently large as compared withthe reference voltage Vref, a current having a maximum amount of N*I₀flows into the transistor Q1, and no current flows into the transistorsQ2 and Q3. As a result, the current having the maximum amount of N*I₀flows as the output current Iout.

Thus, the element circuit 11 with a simple structure including bipolartransistors, as shown in FIG. 9, in which the amount of the outputcurrent Iout can be varied from I₀ to N*I₀ with a change in the controlvoltage Vcont can be manufactured. FIG. 10 is a circuit diagram showingthe details of another example of the element circuit. The bipolartransistors Q1 to Q3 included in the element circuit 11 shown in FIG. 9are replaced by MOSFETs Q1 to Q3, and the MOSFETs Q2 and Q3 areconstructed so that the ratio of the gate widths of the MOSFETs Q2 andQ3 is 1:N−1. The structures of other components and the operation of theelement circuit are the same as those shown in FIG. 9, and the elementcircuit 11 can be manufactured so as to have the structure of FIG. 10.

Embodiment 3

FIG. 11 is a circuit diagram showing the details of an element circuitin accordance with embodiment 3 of the present invention, and shows thedetails of the element circuit 11 of FIG. 2. In the figure, referencesymbol I₀ denotes a constant current source that generates a currenthaving a constant amount of I₀, reference symbol Q11 denotes a bipolartransistor (simply referred to as a transistor on and also referred toas a first transistor from here) having a collector connected to theconstant current source I₀, reference symbol Q12 denotes a transistor(i.e., a second transistor) which constitutes a current mirror circuittogether with the transistor Q11, and reference symbol Q13 denotes atransistor (i.e., a third transistor) which constitutes a current mirrorcircuit together with the transistor Q11, the transistor Q13 having acollector connected to an output current terminal Iout.

Reference symbol Q14 denotes a transistor (i.e., a fourth transistor) towhich a reference voltage Vref is supplied, the transistor Q14 having acollector connected to a power supply Vcc, and reference symbol Q15denotes a transistor (i.e., a fifth transistor) to which a controlvoltage Vcont is supplied and which constitutes a differential pairtogether with the transistor Q14, the emitters of the transistors Q15and Q14 being connected in common to the collector of the transistorQ12.

Reference symbol Q16 denotes a transistor having an emitter connected tothe power supply Vcc and a collector connected to the collector of thetransistor Q15, reference symbol Q17 denotes a transistor having anemitter connected to the power supply Vcc, the transistor Q17constituting a current mirror circuit with the transistor Q16, referencesymbol Q18 denotes a transistor having a collector connected to thecollector of the transistor Q17, and reference symbol Q19 denotes atransistor having a collector connected to the current output terminalIout, the transistor Q19 constituting a current mirror circuit with thetransistor Q18. The transistors Q16 to Q19 constitute a transistorcircuit network.

Next, the operation of the element circuit in accordance with embodiment3 of the present invention will be explained.

In FIG. 11, the current mirror circuits constituted by the transistorsQ11 to Q13 generate both a current having an amount corresponding to therate of the emitter area of the transistor Q12 to that of the transistorQ11 and a current having an amount corresponding to the rate of theemitter area of the transistor Q13 to that of the transistor Q11, whichflow through the transistors Q12 and Q13, respectively, from theconstant current having an amount of I₀ flowing through the currentsource I₀.

A current which flows through the transistor Q12 flows into thetransistor Q12 from the differential pair constituted by the transistorsQ14 and Q15, and is separated into currents respectively flowing throughthe transistors Q14 and Q15 according to the potential differencebetween the reference voltage Vref and the control voltage Vcont.

When the control voltage Vcont is sufficiently small as compared withthe reference voltage Vref, the current I₁₂ is all made to flow from thetransistor Q14 to the transistor Q12 and therefore no current flowsthrough the transistor Q15. In contrast, when the control voltage Vcontis sufficiently large as compared with the reference voltage Vref, thecurrent I₁₂ is all made to flow from the transistor Q15 to thetransistor Q12 and therefore I₁₅ becomes equal to I₁₂.

Both the current mirror circuit constituted by the transistors Q16 andQ17 and the current mirror circuit constituted by the transistors Q18and Q19 generate a current 11 that flows through the transistor Q19 fromthe current I₁₅ that flows into the transistor Q15, the ratio betweenthe currents I₁₉ and the current I₁₅ being associated with both theratio between the emitter areas of the transistors Q16 and Q17 and theratio between the emitter areas of the transistors Q18 and Q19.

When the current I₁₉ flowing through the transistor Q19 has a maximumvalue, the ratio between the emitter areas of the transistors Q12 andQ13, and the ratio among the transistors Q16 to Q19 included in thetransistor circuit network are set so that the ratio of the current I₁₉and the current I₁₃ which flows into the transistor Q13 becomes equal toN−1:1 (N−1 is a ratio at which an output current of the element circuitincreases). In this case, when the control voltage Vcont is sufficientlysmall as compared with the reference voltage Vref, the current I₁₃=I₀flows as the output current Iout, whereas when the control voltage Vcontis sufficiently large as compared with the reference voltage Vref, acurrent having an amount N*I₀ which is the sum of the currentI₁=(N−1)*I₀ and the current I₁₃=I₀ flows as the output current Iout.

More concretely, the ratio between the emitter areas of the transistorsQ12 and Q13 and the ratio among the emitter areas of the transistors Q16to Q19 included in the transistor circuit network simply needs to bedefined according to the following equation (2):Q 12*Q 17*Q 19/Q 13*Q 16*Q 18=N−1  (2)

Thus, the element circuit 11 with a simple structure including bipolartransistors, as shown in FIG. 11, in which the output current Iout canbe varied from I₀ to N*I₀ with a change in the control voltage Vcont canbe manufactured.

FIG. 12 is a circuit diagram showing the details of another example ofthe element circuit. The bipolar transistors Q11 to Q19 included in theelement circuit 11 shown in FIG. 11 are replaced by MOSFETs Q11 to Q19,and the MOSFETs Q12 and Q13 and the MOSFETs Q16 and Q19 included in thetransistor circuit network are constructed so that they have appropriategate widths. The structures of other components and the operation of theelement circuit are the same as those shown in FIG. 11, and the elementcircuit 11 can be manufactured so as to have the structure of FIG. 12.

Embodiment 4

FIG. 13 is a block diagram showing an element circuit in accordance withembodiment 4 of the present invention. In the figure, reference numeral21 denotes the element circuit. FIG. 14 is a characteristics figureshowing control voltage vs. output current characteristics of theelement circuit.

FIG. 15 is a block diagram showing a variable gain amplifier. In thefigure, reference symbols 21 ₁ to 21 _(M) denote M element circuits,respectively, and reference symbol I₀ denotes a constant current sourcethat makes a current having a constant amount of I₀ flow into thevariable gain amplifier. FIG. 16 is a characteristics figure showingcontrol voltage vs. output current characteristics of the variable gainamplifier. The other structure of the variable gain amplifier is thesame as that of the variable gain amplifier shown in FIG. 4.

Next, the operation of the variable gain amplifier in accordance withembodiment 4 of the present invention will be explained.

As shown in FIG. 13, the element circuit 21 has a signal input which isan input current Iin and a signal output which is an output currentIout. In the element circuit 21, a reference voltage Vref and a controlvoltage Vcont are disposed as power supplies.

The element circuit 21 has certain control voltage vs. output currentcharacteristics in which the amount of the output current Iout variesfrom Iin to N*Iin with a predetermined change Vr in the control voltageVcont, that is, the rate of increase in the output current is N−1 whenthe control voltage Vcont can be varied with-respect to the referencevoltage Vref, as shown in FIG. 14.

As shown in FIG. 15, M element circuits 21, i.e., M element circuits 21₁ to 21 _(M) are cascaded-connected in the variable gain amplifier, andthe constant current I₀ is supplied, as the input current Iin, to thefirst-stage one of the plurality of element circuits. Furthermore,voltages that increase in steps of the predetermined voltage change Vrin the control voltage are respectively supplied to the plurality ofelement circuits 21 ₁ to 21 _(M) as their reference voltages Vref1 toVrefM. That is, (VrefM)−(VrefM−1)=Vr. The variable control voltage Vcontis supplied in common to the plurality of element circuits 21 ₁ to 21_(M).

The variable gain control of the amplifier 3 is carried out based on theoutput current Iout from the element circuit 21 _(M) located at the laststage of the plurality of element circuits.

As a result, as shown in FIG. 16, the variable gain amplifier hascontrol voltage vs. output current characteristics in which the amountof the output current Iout varies from I₀, via N*I₀, N²*I₀, . . . , toN^(M*I) ₀ and is approximately expressed by an exponential function withrespect to the voltage change Vr in the control voltage Vcont. When thegain of the amplifier 3 is expressed in a logarithm, the gain of theamplifier 3 can be linearly controlled with respect to the controlvoltage Vcont.

Thus, since the variable gain amplifier does not use exponentialcharacteristics of transistors included in the variable gain amplifier,any change in the characteristics of the variable gain amplifier due tovariations in manufacturing of the transistors can be suppressed.

By setting the number of element circuits included in the variable gainamplifier to an appropriate number and generating the plurality ofreference voltages Vrefl to VrefM with a high degree of precision, theslope of the control voltage vs. output current characteristics curvecan be hardly changed regardless of variations in manufacturing of thetransistors in the whole of the variable gain amplifier, and any changein the characteristics of the variable gain amplifier can be suppressed.

When the plurality of element circuits are thus cascade-connected, sinceat a connecting point between any two adjacent element circuits theircontrol voltage vs. output current characteristics curves are shifted inupward and downward directions due to the temperature dependence of thecontrol voltage vs. output current characteristics of each of the twoadjacent element circuits, respectively, the upward and downward shiftedparts of the control voltage vs. output current characteristics curvescan be compensated. Therefore, the temperature dependence can becompensated.

Embodiment 5

FIG. 17 is a circuit diagram showing the details of an element circuitin accordance with embodiment 5 of the present invention, and shows thedetails of the element circuit 21 of FIG. 13. In the figure, referencesymbol Q21 denotes a bipolar transistor (simply referred to as atransistor and also referred to as a fourth transistor) having acollector into which an input current Iin flows, and reference symbolQ22 denotes a transistor (i.e., a fifth transistor) having a collectorconnected in common to the emitters of transistors Q1 to Q3, thetransistor Q22 constituting a current mirror circuit with the transistorQ21.

Reference symbol Q23 denotes a transistor having an emitter connected toa power supply Vcc and a collector connected in common to the collectorsof the transistors Q1 and Q2, and reference symbol Q24 denotes atransistor having an emitter connected to the power supply Vcc, and acollector from which an output current Iout flows, the transistor Q24constituting a current mirror circuit with the transistor Q23. Thetransistors Q23 and Q24 constitute an output current circuit. The otherstructure of the element circuit is the same as that of the elementcircuit shown in FIG. 9.

Next, the operation of the element circuit in accordance with embodiment5 of the present invention will be explained.

In FIG. 17, the transistors Q21 and Q22 constitute a current mirrorcircuit, and the radio between their emitter areas is defined so that acurrent having an amount of N*Iin flows through the transistor Q22 withrespect to the input current Iin.

When a control voltage Vcont is sufficiently small as compared with areference voltage Vref, no current flows into the transistor Q1. Inaddition, since the transistors Q2 and Q3 constitute a current mirrorcircuit in which the ratio between their emitter areas is defined as1:N−1, a current having an amount of Iin flows into the transistor Q2and a current having an amount of (N−1)*Iin flows into the transistorQ3. As a result, a current having an amount of Iin flows through thetransistor Q23, and a current having an amount of Iin flows through thetransistor Q24 which constitutes a current mirror circuit together withthe transistor Q23, as the output current Iout.

In contrast, when the control voltage Vcont is sufficiently large ascompared with the reference voltage Vref, a current having a maximumamount of N*Iin flows into the transistor Q1, and no current flows intothe transistors Q2 and Q3. As a result, a current having an amount ofN*Iin flows through the transistor Q23, and a current having an amountof N*Iin flows through the transistor Q24 which constitutes a currentmirror circuit together with the transistor Q23, as the output currentIout.

Thus, the element circuit 21 with a simple structure including bipolartransistors, as shown in FIG. 17, in which the amount of the outputcurrent Iout can be varied from Iin to N*Iin with a change in thecontrol voltage Vcont can be manufactured.

The ratio between the emitter areas of the transistors Q21 and Q22 isdefined as 1:N, as previously mentioned. As an alternative, the ratioamong the emitter areas of the transistors Q21 to Q24 can be set so thatQ22*Q24/Q21*Q23=N is satisfied.

FIG. 18 is a circuit diagram showing the details-of another example ofthe element circuit. The bipolar transistors Q1 to Q3 and Q21 to Q24included in the element circuit 21 shown in FIG. 17 are replaced byMOSFETs Q1 to Q3 and Q21 to Q24, and the MOSFETs Q2 and Q3 and theMOSFETs Q21 to Q24 are constructed so that the ratio between the gatewidths of the MOSFETs Q2 and Q3 is defined as 1:N−1 and the ratio amongthe gate widths of the MOSFETs Q21 to Q24 is defined so thatQ22*Q24/Q21*Q23=N is satisfied. The structures of other components andthe operation of the element circuit are the same as those shown in FIG.17, and the element circuit 21 can be manufactured so as to have thestructure of FIG. 18.

Embodiment 6

FIG. 19 is a circuit diagram showing the details of an element circuitin accordance with embodiment 6 of the present invention, and shows thedetails of an example of the element circuit 21 of FIG. 13. As shown inthe figure, the element circuit is so constructed so that an inputcurrent Iin flows into the collector of a transistor Q11.

Reference symbol Q31 denotes a transistor having an emitter connected toa power supply Vcc and a collector connected in common to the collectorsof transistors Q13 and Q19, and reference symbol Q32 denotes atransistor having an emitter connected to the power supply Vcc, and acollector connected to an output current terminal Iout, the transistorQ32 constituting a current mirror circuit with the transistor Q31. Thetransistors Q31 and Q32 constitute an output current circuit. The otherstructure of the element circuit is the same as that of the elementcircuit shown in FIG. 11.

Next, the operation of the element circuit in accordance with embodiment6 of the present invention will be explained.

In FIG. 19, when the input current Iin flows into the transistor Q11,the transistors Q12 and Q13 make a current having an amountcorresponding to the rate between the emitter 20 area of the transistorQ12 to that of the transistor Q11 and a current having an amountcorresponding to the rate between the emitter area of the transistor Q13to that of the transistor Q11 flow therethrough, respectively.

As a result, as previously mentioned in embodiment 3, when a controlvoltage Vcont is sufficiently small as compared with a reference voltageVref, a current I₁₃=Iin flows through the transistor Q3, whereas whenthe control voltage Vcont is sufficiently large as compared with thereference voltage Vref, a current having an amount of N*Iin which is thesum of a current I₁₉=(N−1)*Iin and the current I₁₃=Iin flows into thetransistor Q31.

Since the transistors Q31 and Q32 constitute a current mirror circuit, acurrent having the same amount as that of the current flowing throughthe transistor Q31 flows as an output current Iout.

Thus, the element circuit 21 with a simple structure including bipolartransistors, as shown in FIG. 19, in which the output current Iout canbe varied from Iin to N*Iin with a change in the control voltage Vcontcan be manufactured.

FIG. 20 is a circuit diagram showing the details of another example ofthe element circuit. The bipolar transistors Q11 to Q19 and Q31 and Q32included in the element circuit 21 shown in FIG. 19 are replaced byMOSFETs Q11 to Q19 and Q31 and Q32, and the gate widths of the MOSFETsQ12 and Q13 are set to appropriate values and the gate widths of theMOSFETs Q16 to Q19 included in the transistor circuit network are set toappropriate values. The structures of other components and the operationof the element circuit are the same as those shown in FIG. 19, and theelement circuit 21 can be manufactured so as to have the structure shownin FIG. 20.

Embodiment 7

FIG. 21 is a block diagram showing an element circuit in accordance withembodiment 7 of the present invention. In the figure, reference numeral31 denotes the element circuit. FIG. 22 is a figure showing controlvoltage vs. gain characteristics of the element circuit.

FIG. 23 is a block diagram showing a variable gain amplifier. In thefigure, reference symbols 31 ₁ to 31 _(M) denote M element circuits,respectively. FIG. 24 is a characteristics figure showing controlvoltage vs. gain characteristics of the variable gain amplifier.

Next, the operation of the variable gain amplifier in accordance withembodiment 7 of the present invention will be explained.

As shown in FIG. 21, the element circuit 31 has a signal input which isan input voltage Vin and a signal output which is an output voltageVout. In the element circuit 31, a reference voltage Vref and a controlvoltage Vcont are disposed as power supplies.

The element circuit 31 has certain control voltage vs. gaincharacteristics in which its gain Gain varies from G₀ to N*G₀ with apredetermined change Vr in the control voltage Vcont, that is, the rateof increase in the gain is N−1 when the control voltage Vcont can bevaried with respect to the reference voltage Vref, as shown in FIG. 22.

As shown in FIG. 23, M element circuits 31, i.e., the M element circuits31 ₁ to 31 _(M) are cascaded-connected in the variable gain amplifier,and an input voltage Vin is supplied to the first-stage one of theplurality of element circuits. Furthermore, voltages that increase insteps of the predetermined voltage change Vr in the control voltage arerespectively supplied to the plurality of element circuits 311 to 31M astheir reference voltages Vref1 to VrefM. That is, (VrefM)−(VrefM−1)=Vr.The variable control voltage Vcont is supplied in common to theplurality of element circuits 311 to 31M, and the output voltage Vout isgenerated by the element circuit 31M located at the last stage of theplurality of element circuits.

As a result, as shown in FIG. 24, the variable gain amplifier hascontrol voltage vs. gain characteristics in which the gain Gain variesfrom G₀ ^(M), via N*G₀ ^(M), N²*G₀ ^(M), . . . , to N^(M)*G₀ ^(M) and isapproximately expressed by an exponential function with respect to thevoltage change Vr in the control voltage Vcont. When the gain isexpressed in a logarithm, the gain can be linearly controlled withrespect to the control voltage Vcont.

Thus, since the variable gain amplifier does not use exponentialcharacteristics of transistors included in the variable gain amplifier,any change in the characteristics of the variable gain amplifier due tovariations in manufacturing of the transistors can be suppressed.

By setting the number of element circuits included in the variable gainamplifier to an appropriate number and generating the plurality ofreference voltages Vrefl to-VrefM with a high degree of precision, theslope of the control voltage vs. gain characteristics curve can behardly changed regardless of variations in manufacturing of thetransistors in the whole of the variable gain amplifier, and any changein the characteristics of the variable gain amplifier can be suppressed.

When the plurality of element circuits are thus cascade-connected, sinceat a connecting point between any two adjacent element circuits theircontrol voltage vs. gain characteristics curves are shifted in upwardand downward directions due to the temperature dependence of the controlvoltage vs. gain characteristics of each of the two element circuits,respectively, the upward and downward shifted parts of the controlvoltage vs. gain characteristics curves can be compensated. Therefore,the temperature dependence can be compensated.

Embodiment 8

FIG. 25 is a circuit diagram showing the details of an element circuitin accordance with embodiment 8 of the present invention, and shows thedetails of an example of the element circuit 31 of FIG. 21. In thefigure, reference symbols R1 and R2 denote resistors, and referencesymbol Q41 denotes a bipolar transistor (referred to as a transistor andalso refereed to as a fourth transistor from here on) having a collectorconnected in common to the emitters of transistors Q1 to Q3, and anemitter connected to the resistor R2, an input voltage Vin being appliedto the transistor Q41.

A power supply Vcc is connected to the collectors of the transistors Q1and Q2 by way of the resistor R1, and is also connected directly to thecollector of the transistor Q3. Furthermore, the element circuit is soconstructed that an output voltage Vout is outputted from between theresistor R1 and the collectors of the transistors Q1 and Q2. The otherstructure of the element circuit is the same as that of the elementcircuit shown in FIG. 9.

Next, the operation of the element circuit in accordance with embodiment8 of the present invention will be explained.

In FIG. 25, a current having an amount corresponding to the inputvoltage Vin flows into the transistor Q41. When a control voltageVcont-is sufficiently small as compared with a reference voltage Vref,no current flows into the transistor Q1. In addition, since thetransistors Q2 and Q3 constitute a current mirror circuit in which theratio between the emitter areas of the transistors is defined as 1:N−1,a current having an amount of Iin flows into the transistor Q2 and acurrent having an amount of (N−1)*Iin flows into the transistor Q3. As aresult, a current having an-amount of Iin flows through the resistor R1,and therefore a voltage of Iin*R1 is generated as the output voltageVout.

When the control voltage Vcont is sufficiently large as compared withthe reference voltage Vref, a current having a maximum amount of N*Iinflows into the transistor Q1, and no current flows into the transistorsQ2 and Q3. As a result, a current having an amount of N*Iin flowsthrough the resistor R1, and a voltage of N*Iin*R1 is generated as theoutput voltage Vout.

Thus, the element circuit 31 with a simple structure including bipolartransistors, as shown in FIG. 25, in which the output voltage Vout canbe varied from Iin*R1 to N*Iin*R1 with a change in the control voltageVcont, that is, the gain can be varied from G0 to N*G0, where Iin*R1 isdefined as the gain G0, with a change in the control voltage Vcont canbe manufactured.

INDUSTRIAL APPLICABILITY

As mentioned above, the variable gain amplifier in accordance with thepresent invention is suitable for suppressing any change in thecharacteristics thereof due the temperature compensation of thecharacteristics and variations in manufacturing of transistors, and forcarrying out a linear gain control operation on the control voltage.

1. A variable gain amplifier comprising: a plurality of element circuitseach of which has two inputs which are a reference voltage and a controlvoltage which can vary with respect to the reference voltage, and eachof which has an output current that increases at a constant rate with apredetermined change in the control voltage, voltages which increase insteps of the predetermined change in the control voltage beingrespectively supplied to said plurality of element circuits as theirreference voltages and the variable control voltage being supplied toeach of said plurality of element circuits; multipliers for multiplyingthe output currents from said plurality of element circuits by oneanother; and an amplifier for carrying out a variable gain amplificationbased on an output current obtained by said multipliers.
 2. The variablegain amplifier according to claim 1, characterized in that each of saidplurality of element circuits includes: a first transistor to which thecontrol voltage is supplied; a second transistor to which the referencevoltage is supplied and which constitutes a differential pair togetherwith said first transistor; and a third transistor to which thereference voltage is supplied and which constitutes a current mirrorcircuit together with said second transistor, a ratio between saidsecond transistor and said third transistor in size being 1:N−1 when theconstant rate at which the output current increases is N−1 (N is anarbitrary number larger than 1), and characterized in that the outputcurrent is output in common from ends of said first and secondtransistors and a constant current source that outputs a maximum outputcurrent is connected in common to other ends of said first through thirdtransistors.
 3. The variable gain amplifier according to claim 1,characterized in that each of said plurality of element circuitsincludes: a first transistor having an end connected to a constantcurrent source; a second transistor which constitutes a current mirrorcircuit together with said first transistor; a third transistor whichconstitutes a current mirror circuit together with said firsttransistor, said third transistor having an end connected to an outputcurrent terminal; a fourth transistor to which the reference voltage issupplied; a fifth transistor to which the control voltage is suppliedand which constitutes a differential pair together with said fourthtransistor, ends of said fifth and fourth transistors being connected incommon to an end of said second transistor; and a transistor circuitnetwork that diverts a part of a current from said output currentterminal in proportion to a current flowing through said fifthtransistor and that makes the part of the current pass therethroughwithout making it flow through said third transistor, and characterizedin that sizes of said second and third transistors and transistorsincluded in said transistor circuit network are determined so that aratio between the diverted part of the current and the current flowingthrough said third transistor is N−1:1 (N−1 is the constant rate atwhich the output current increases and N is an arbitrary number largerthan 1) when the diverted part of the current has a maximum value.
 4. Avariable gain amplifier comprising: a plurality of cascade-connectedelement circuits each of which has two inputs which are a referencevoltage and a control voltage which can vary with respect to thereference voltage, and each of which has an output current thatincreases at a constant rate with a predetermined change in the controlvoltage, an input current being supplied to a first-stage one of saidplurality of element circuits, voltages which increase in steps of thepredetermined change in the control voltage being respectively suppliedto said plurality of element circuits as their reference voltages, andthe variable control voltage being supplied to each of said plurality ofelement circuits; and an amplifier for carrying out a variable gainamplification based on an output current from said plurality of elementcircuits.
 5. The variable gain amplifier according to claim 1,characterized in that each of said plurality of element circuitsincludes: a first transistor to which the control voltage is supplied; asecond transistor to which the reference voltage is supplied and whichconstitutes a differential pair together with said first transistor; athird transistor to which the reference voltage is supplied and whichconstitutes a current mirror circuit together with said secondtransistor, a ratio between said second transistor and said thirdtransistor in size being 1:N−1 when the constant rate at which theoutput current increases is N−1 (N is an arbitrary number larger than1); a fourth transistor having an end into which an input current flows;a fifth transistor that has an end connected in common to ends of saidfirst through third transistors, and that constitutes a currentmirror-circuit together with said fourth transistor; and an outputcurrent circuit connected in common to other ends of said first andsecond transistors.
 6. The variable gain amplifier according to claim 1,characterized in that each of said plurality of element circuitsincludes: a first transistor having an end into which an input currentflows; a second transistor which constitutes a current mirror circuittogether with said first transistor; a third transistor whichconstitutes a current mirror circuit together with said firsttransistor, said third transistor having an end connected to an outputcurrent circuit; a fourth transistor to which the reference voltage issupplied; a fifth transistor to which the control voltage is suppliedand which constitutes a differential pair together with said fourthtransistor, ends of said fifth and fourth transistors being connected incommon to an end of said second transistor; and a transistor circuitnetwork that diverts a part of a current from said output currentcircuit in proportion to a current flowing through said fifth transistorand that makes the part of the current pass therethrough without makingit flow through said third transistor, and characterized in that sizesof said second and third transistors and transistors included in saidtransistor circuit network are determined so that a ratio between thediverted part of the current and the current flowing through said thirdtransistor is N−1:1 (N−1 is the constant rate at which the outputcurrent increases and N is an arbitrary number larger than 1) when thediverted part of the current has a maximum value.
 7. A variable gainamplifier comprising: a plurality of cascade-connected element circuitseach of which has two inputs which are a reference voltage and a controlvoltage which can vary with respect to the reference voltage, and eachof which has a gain that increases at a constant rate with apredetermined change in the control voltage, an input voltage beingsupplied to a first-stage one of said plurality of element circuits,voltages which increase in steps of the predetermined change in thecontrol voltage being respectively supplied to said plurality of elementcircuits as their reference voltages, the variable control voltage beingsupplied to each of said plurality of element circuits, and an outputvoltage being generated by a last-stage one of said plurality of elementcircuits.
 8. The variable gain amplifier according to claim 7,characterized in that each of said plurality of element circuitsincludes: a first transistor to which the control voltage is supplied; asecond transistor to which the reference voltage is supplied and whichconstitutes a differential pair together with said first transistor; athird transistor to which the reference voltage is supplied and whichconstitutes a current mirror circuit together with said secondtransistor, a ratio between said second transistor and said thirdtransistor in size being 1:N−1 when the constant rate at which the gainincreases is N−1 (N is an arbitrary number larger than 1); a fourthtransistor to which an input voltage is supplied, said fourth transistorhaving an end connected in common to ends of said first through thirdtransistors; and a resistor connected between other ends of said firstand second transistors and a power supply, and characterized in thateach of said plurality of element circuits generates an output voltagefrom between said resistor and the other ends of said first and secondtransistors.